U.S. patent application number 13/200307 was filed with the patent office on 2012-01-26 for method of producing nano-scaled inorganic platelets.
Invention is credited to Bor Z. Jang, Aruna Zhamu.
Application Number | 20120021293 13/200307 |
Document ID | / |
Family ID | 39969717 |
Filed Date | 2012-01-26 |
United States Patent
Application |
20120021293 |
Kind Code |
A1 |
Zhamu; Aruna ; et
al. |
January 26, 2012 |
Method of producing nano-scaled inorganic platelets
Abstract
The present invention provides a method of exfoliating a layered
material (e.g., transition metal dichalcogenide) to produce
nano-scaled platelets having a thickness smaller than 100 nm,
typically smaller than 10 nm. The method comprises (a) dispersing
particles of a non-graphite laminar compound in a liquid medium
containing therein a surfactant or dispersing agent to obtain a
stable suspension or slurry; and (b) exposing the suspension or
slurry to ultrasonic waves at an energy level for a sufficient
length of time to produce separated nano-scaled platelets. The
nano-scaled platelets are candidate reinforcement fillers for
polymer nanocomposites.
Inventors: |
Zhamu; Aruna; (Centerville,
OH) ; Jang; Bor Z.; (Centerville, OH) |
Family ID: |
39969717 |
Appl. No.: |
13/200307 |
Filed: |
September 23, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11800728 |
May 8, 2007 |
7824651 |
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13200307 |
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Current U.S.
Class: |
429/231.5 ;
423/509; 423/561.1; 502/215; 502/216; 502/219; 502/220; 502/5;
977/775; 977/888; 977/948 |
Current CPC
Class: |
C01B 32/225
20170801 |
Class at
Publication: |
429/231.5 ;
502/5; 423/561.1; 423/509; 502/220; 502/216; 502/219; 502/215;
977/775; 977/948; 977/888 |
International
Class: |
H01M 4/134 20100101
H01M004/134; C01G 39/06 20060101 C01G039/06; C01G 35/00 20060101
C01G035/00; C01G 41/00 20060101 C01G041/00; B01J 27/051 20060101
B01J027/051; B01J 27/04 20060101 B01J027/04; B01J 27/047 20060101
B01J027/047; B01J 27/057 20060101 B01J027/057; B01J 37/34 20060101
B01J037/34; C01B 19/04 20060101 C01B019/04 |
Goverment Interests
[0002] This invention is based on the research result of a US
Department of Energy (DoE) Small Business Innovation Research
(SBIR) project. The US government has certain rights on this
invention.
Claims
1. A method of exfoliating a layered material to produce separated
nano-scaled platelets having a thickness smaller than 100 nm, said
method comprising: a) dispersing particles of a layered material in
a liquid medium containing therein a surfactant or dispersing agent
to produce a suspension or slurry, wherein said layered material is
selected from a transition metal dichalcogenide, MoS.sub.2,
TaS.sub.2, WS.sub.2, MoSe.sub.2, TaSe.sub.2, WSe.sub.2, or a
combination thereof; and b) exposing said suspension or slurry to
ultrasonication at an energy level for a sufficient length of time
to produce said separated nano-scaled platelets.
2. A method of exfoliating a non-graphite layered material to
produce separated nano-scaled platelets having a thickness smaller
than 100 nm, said method comprising: a) dispersing particles of a
layered material in a liquid medium containing therein a surfactant
or dispersing agent to produce a suspension or slurry, wherein said
layered material does not include graphite, natural graphite,
synthetic graphite, highly oriented pyrolytic graphite, graphite
oxide, graphite intercalated with a non-halogen intercalate,
graphite fiber, graphitic nano-fiber; and b) exposing said
suspension or slurry to ultrasonication at an energy level for a
sufficient length of time to produce said separated nano-scaled
platelets.
3. The method of claim 1 wherein said ultrasonication step is
conducted at a temperature lower than 100.degree. C.
4. The method of claim 2 wherein said ultrasonication step is
conducted at a temperature lower than 100.degree. C.
5. The method of claim 1 wherein said energy level is greater than
80 watts.
6. The method of claim 2 wherein said energy level is greater than
80 watts.
7. The method of claim 1 wherein said ultrasonication step is
followed by a mechanical shearing treatment selected from air
milling, ball milling, rotating blade shearing, or a combination
thereof.
8. The method of claim 2 wherein said ultrasonication step is
followed by a mechanical shearing treatment selected from air
milling, ball milling, rotating blade shearing, or a combination
thereof.
9. The method of claim 1 wherein said liquid medium comprises
water, organic solvent, alcohol, a monomer, an oligomer, or a
resin.
10. The method of claim 1 wherein said liquid medium comprises
water, organic solvent, alcohol, a monomer, an oligomer, or a
resin.
11. The method of claim 1 wherein said platelets comprise
single-layer or double-layer platelets.
12. The method of claim 2 wherein said platelets comprise
single-layer or double-layer platelets.
13. The method of claim 1 wherein said surfactant or dispersing
agent is selected from the group consisting of anionic surfactants,
nonionic surfactants, cationic surfactants, amphoteric surfactants,
silicone surfactants, fluoro-surfactants, polymeric surfactants,
sodium hexametaphosphate, sodium lignosulphonate, poly (sodium
4-styrene sulfonate), sodium dodecylsulfate, sodium sulfate, sodium
phosphate, sodium sulfonate, and combinations thereof.
14. The method of claim 2 wherein said surfactant or dispersing
agent is selected from the group consisting of anionic surfactants,
nonionic surfactants, cationic surfactants, amphoteric surfactants,
silicone surfactants, fluoro-surfactants, polymeric surfactants,
sodium hexametaphosphate, sodium lignosulphonate, poly (sodium
4-styrene sulfonate), sodium dodecylsulfate, sodium sulfate, sodium
phosphate, sodium sulfonate, and combinations thereof.
15. A method of exfoliating a non-graphite layered material to
produce separated nano-scaled platelets having a thickness smaller
than 100 nm, said method comprising: a) dispersing particles of a
layered material in a liquid medium to produce a suspension or
slurry, wherein said layered material does not include graphite,
natural graphite, synthetic graphite, highly oriented pyrolytic
graphite, graphite oxide, graphite intercalated with a non-halogen
intercalate, graphite fiber, graphitic nano-fiber; and b) exposing
said suspension or slurry to ultrasonication at an energy level for
a sufficient length of time to produce said separated nano-scaled
platelets.
16. The method of claim 15, wherein said layered material is
selected from a transition metal dichalcogenide, MoS.sub.2,
TaS.sub.2, WS.sub.2, MoSe.sub.2, TaSe.sub.2, WSe.sub.2, or a
combination thereof.
17. A lithium-ion battery electrode containing said separated
nano-scaled platelets of claim 1 as an electrode material.
18. A lithium-ion battery electrode containing said separated
nano-scaled platelets of claim 2 as an electrode material.
19. A hydro-desulfurization catalyst containing one of said
separated nano-scaled platelets of claim 1 as a catalyst
material.
20. A hydro-desulfurization catalyst containing one of said
separated nano-scaled platelets of claim 2 as a catalyst material.
Description
[0001] This is a divisional application of Aruna Zhamu, et al,
"Method of Producing Exfoliated Graphite, Flexible Graphite, and
Nano-Scaled Graphene Plates," U.S. patent application Ser. No.
11/800,728 (May 8, 2007); now U.S. Pat. No. 7,824,651 (Nov. 2,
2010).
FIELD OF THE INVENTION
[0003] The present invention relates to a method of exfoliating and
separating graphite, graphite oxide, and other laminar compounds to
produce nano-scaled platelets (particularly nano-scaled graphene
platelets, NGPs) and re-compressed flexible graphite.
BACKGROUND
[0004] Carbon is known to have four unique crystalline structures,
including diamond, graphite, fullerene and carbon nano-tubes. The
carbon nano-tube (CNT) refers to a tubular structure grown with a
single wall or multi-wall, which can be conceptually obtained by
rolling up a graphene sheet or several graphene sheets to form a
concentric hollow structure. A graphene sheet is composed of carbon
atoms occupying a two-dimensional hexagonal lattice. Carbon
nano-tubes have a diameter on the order of a few nanometers to a
few hundred nanometers. Carbon nano-tubes can function as either a
conductor or a semiconductor, depending on the rolled shape and the
diameter of the tubes. Its longitudinal, hollow structure imparts
unique mechanical, electrical and chemical properties to the
material. Carbon nano-tubes are believed to have great potential
for use in field emission devices, hydrogen fuel storage,
rechargeable battery electrodes, and as composite
reinforcements.
[0005] However, CNTs are extremely expensive due to the low yield
and low production rates commonly associated with all of the
current CNT preparation processes. The high material costs have
significantly hindered the widespread application of CNTs. Rather
than trying to discover much lower-cost processes for nano-tubes,
we have worked diligently to develop alternative nano-scaled carbon
materials that exhibit comparable properties, but can be produced
in larger quantities and at much lower costs. This development work
has led to the discovery of processes for producing individual
nano-scaled graphite planes (individual graphene sheets) and stacks
of multiple nano-scaled graphene sheets, which are collectively
called "nano-scaled graphene plates (NGPs)." NGPs could provide
unique opportunities for solid state scientists to study the
structures and properties of nano carbon materials. The structures
of these materials may be best visualized by making a longitudinal
scission on the single-wall or multi-wall of a nano-tube along its
tube axis direction and then flattening up the resulting sheet or
plate. Studies on the structure-property relationship in isolated
NGPs could provide insight into the properties of a fullerene
structure or nano-tube. Furthermore, these nano materials could
potentially become cost-effective substitutes for carbon nano-tubes
or other types of nano-rods for various scientific and engineering
applications. The electronic, thermal and mechanical properties of
NGP materials are expected to be comparable to those of carbon
nano-tubes; but NGP will be available at much lower costs and in
larger quantities.
[0006] Direct synthesis of the NGP material had not been possible,
although the material had been conceptually conceived and
theoretically predicted to be capable of exhibiting many novel and
useful properties. Jang and Huang have provided an indirect
synthesis approach for preparing NGPs and related materials [B. Z.
Jang and W. C. Huang, "Nano-scaled Graphene Plates," U.S. Pat. No.
7,071,258 (Jul. 4, 2006)]. In most of the prior art methods for
making separated graphene platelets, the process begins with
intercalating lamellar graphite flake particles with an expandable
intercalation agent (intercalant), followed by thermally expanding
the intercalant to exfoliate the flake particles. In some methods,
the exfoliated graphite is then subjected to air milling, ball
milling, or ultrasonication for further flake separation and size
reduction. Conventional intercalation methods and recent attempts
to produce exfoliated products or separated platelets are given in
the following representative references: [0007] 1. J. W. Kraus, et
al., "Preparation of Vermiculite Paper," U.S. Pat. No. 3,434,917
(Mar. 25, 1969). [0008] 2. L. C. Olsen, et al., "Process for
Expanding Pyrolytic Graphite," U.S. Pat. No. 3,885,007 (May 20,
1975). [0009] 3. A. Hirschvogel, et al., "Method for the Production
of Graphite-Hydrogensulfate," U.S. Pat. No. 4,091,083 (May 23,
1978). [0010] 4. T. Kondo, et al., "Process for Producing Flexible
Graphite Product," U.S. Pat. No. 4,244,934 (Jan. 13, 1981). [0011]
5. R. A. Greinke, et al., "Intercalation of Graphite," U.S. Pat.
No. 4,895,713 (Jan. 23, 1990). [0012] 6. F. Kang, "Method of
Manufacturing Flexible Graphite," U.S. Pat. No. 5,503,717 (Apr. 2,
1996). [0013] 7. F. Kang, "Formic Acid-Graphite Intercalation
Compound," U.S. Pat. No. 5,698,088 (Dec. 16, 1997). [0014] 8. P. L.
Zaleski, et al. "Method for Expanding Lamellar Forms of Graphite
and Resultant Product," U.S. Pat. No. 6,287,694 (Sep. 11, 2001).
[0015] 9. J. J. Mack, et al., "Chemical Manufacture of
Nanostructured Materials," U.S. Pat. No. 6,872,330 (Mar. 29,
2005).
[0016] However, these previously invented methods had a serious
drawback. Typically, exfoliation of the intercalated graphite
occurred at a temperature in the range of 800.degree. C. to
1,050.degree. C. At such a high temperature, graphite could undergo
severe oxidation, resulting in the formation of graphite oxide,
which has much lower electrical and thermal conductivities compared
with un-oxidized graphite. In our recent studies, we have
surprisingly observed that the differences in electrical
conductivity between oxidized and non-oxidized graphite could be as
high as several orders of magnitude. It may be noted that the
approach proposed by Mack, et al. [e.g., Ref. 9, U.S. Pat. No.
6,872,330] is also a low temperature process. However, it involves
intercalating graphite with potassium melt, which must be carefully
conducted in a vacuum or extremely dry glove box environment since
pure alkali metals like potassium and sodium are extremely
sensitive to moisture and pose an explosion danger. This process is
not amenable to mass production of nano-scaled platelets.
[0017] To address these issues, we have recently developed several
processes for producing nano-scaled platelets, as summarized in
several co-pending patent applications [Refs. 10-13]: [0018] 10.
Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, "Process for Producing
Nano-scaled Platelets and Nanocomposites," U.S. patent Pending Ser.
No. 11/509,424 (Aug. 25, 2006). [0019] 11. Bor Z. Jang, Aruna
Zhamu, and Jiusheng Guo, "Mass Production of Nano-scaled Platelets
and Products," U.S. patent Pending Ser. No. 11/526,489 (Sep. 26,
2006). [0020] 12. Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo,
"Method of Producing Nano-scaled Graphene and Inorganic Platelets
and Their Nanocomposites," U.S. patent Pending Ser. No. 11/709,274
(Feb. 22, 2007). [0021] 13. Aruna Zhamu, JinJun Shi, Jiusheng Guo,
and Bor Z. Jang, "Low-Temperature Method of Producing Nano-scaled
Graphene Platelets and Their Nanocomposites," U.S. patent Pending
Ser. No. 11/787,442 (Apr. 17, 2007).
[0022] References [10,11] are related to processes that entail a
pressurized gas-induced intercalation procedure to obtain a
tentatively intercalated layered compound and a heating and/or gas
releasing procedure to generate a supersaturation condition for
inducing exfoliation of the layered compound. Tentative
intercalation implies that the intercalating gas molecules are
forced by a high gas pressure to reside tentatively in the
interlayer spaces. Once the intercalated material is exposed to a
thermal shock, these gas molecules induce a high gas pressure that
serves to push apart neighboring layers. Reference [12] is related
to a halogen intercalation procedure, followed by a relatively
low-temperature exfoliation procedure. No strong acid like sulfuric
acid or nitric acid is used in this process (hence, no SO.sub.2 or
NO.sub.2 emission) and halogen can be recycled and re-used. This is
an environmentally benign process.
[0023] Reference [13] provides a low-temperature method of
exfoliating a layered material to produce separated nano-scaled
platelets. The method entails exposing a graphite intercalation
compound to an exfoliation temperature lower than 650.degree. C.
for a duration of time sufficient to at least partially exfoliate
the layered graphite without incurring a significant level of
oxidation. This is followed by subjecting the partially exfoliated
graphite to a mechanical shearing treatment to produce separated
platelets. The key feature of this method is the exfoliation at low
temperature to avoid oxidation of graphite. This was based on the
finding that no oxidation of graphite occurs at 650.degree. C. or
lower for a short duration of heat exposure (e.g., shorter than 45
seconds) and at 350.degree. C. or lower for a slightly longer
duration of heat exposure (e.g., 2 minutes). The resulting NGPs
exhibit very high electrical conductivity, much higher than that of
NGPs obtained with exfoliation at higher temperatures.
[0024] In all of aforementioned prior art methods and our
co-pending applications, the process begins with intercalation of
graphite, followed by gas pressure-induced exfoliation of the
resulting intercalated graphite. The gas pressure is generated by
heating and/or chemical reaction. However, intercalation by a
chemical (e.g., an acid) is not desirable. Exfoliation by heat can
put graphite at risk of oxidation. After exfoliation, an additional
mechanical shear treatment is needed to separate the exfoliated
graphite into isolated platelets. In essence, every one of these
processes involves three separate steps, which can be tedious and
energy-intensive.
[0025] It is therefore an object of the present invention to
provide a simpler, faster, and less energy-intensive method of
expanding a laminar (layered) compound or element, such as graphite
and graphite oxide (partially oxidized graphite), to produce
exfoliated graphite and graphite oxide and nano-scaled graphite and
graphite oxide flakes or platelets.
[0026] It is another object of the present invention to provide a
convenient method of exfoliating a laminar material to produce
nano-scaled platelets (platelets with a thickness smaller than 100
nm and mostly smaller than 10 nm) without the intercalation step
and, hence, without the utilization of an intercalant such as
sulfuric acid.
[0027] It is yet another object of the present invention to provide
a convenient method of exfoliating a laminar material to produce
nano-scaled platelets without involving a heat- or chemical
reaction-induced gas pressurization step.
[0028] Another object of the present invention is to provide an
effective and safe method of mass-producing nano-scaled
platelets.
[0029] It is still another object of the present invention to
provide a method of producing nano-scaled platelets that can be
readily dispersed in a liquid to form a nanocomposite
structure.
SUMMARY OF THE INVENTION
[0030] The present invention provides a method of exfoliating a
layered material (e.g., graphite and graphite oxide) to produce
nano-scaled platelets having a thickness smaller than 100 nm,
typically smaller than 10 nm. The method comprises (a) dispersing
graphite or graphite oxide particles in a liquid medium containing
therein a surfactant or dispersing agent to obtain a suspension or
slurry; and (b) exposing the suspension or slurry to ultrasonic
waves (a process commonly referred to as ultrasonication) at an
energy level for a sufficient length of time to produce the
separated nano-scaled platelets.
[0031] Preferably, the ultrasonication step is conducted at a
temperature lower than 100.degree. C. The energy level is typically
greater than 80 watts. Optionally, the ultrasonication step may be
followed by a mechanical shearing treatment selected from air
milling, ball milling, rotating-blade shearing, or a combination
thereof to further separate the platelets and/or reduce the size of
the platelets. The liquid medium may comprise water, organic
solvent, alcohol, a monomer, an oligomer, or a resin. The layered
graphite material could be natural graphite, synthetic graphite,
highly oriented pyrolytic graphite, graphite oxide, graphite fiber,
graphite nano-fiber, or a combination thereof.
[0032] Certain nano-scaled platelets (e.g., graphite oxides) are
hydrophilic in nature and, therefore, can be readily dispersed in
selected polar solvents (e.g., water). Hence, this invented method
intrinsically involves dispersing the platelets in a liquid to form
a suspension or in a monomer- or polymer-containing solvent to form
a nanocomposite precursor suspension. This suspension can be
converted to a mat or paper (e.g., by following a paper-making
process). The nanocomposite precursor suspension may be converted
to a nanocomposite solid by removing the solvent or polymerizing
the monomer. In the case of graphite oxide platelets, the method
may further include a step of partially or totally reducing the
graphite oxide (after the formation of the suspension) to become
graphite (serving to recover at least partially the high
conductivity that a pristine graphite would have).
[0033] It may be noted that ultrasonication was used to
successfully separate graphite flakes after exfoliation. Examples
are given in Sakawaki, et al. ("Foliated Fine Graphite Particles
and Method for Preparing Same," U.S. Pat. No. 5,330,680, Jul. 19,
1994); and Mack, et al. (U.S. Pat. No. 6,872,330, Mar. 29, 2005).
However, there has been no report on the utilization of ultrasonic
waves in directly exfoliating graphite or graphite oxide (with or
without intercalation) and, concurrently, separating exfoliated
particles into isolated or separated graphite flakes or platelets
with a thickness less than 100 nm. Those who are skilled in the art
of expandable graphite, graphite exfoliation, and flexible graphite
have hitherto firmly believed that graphite or other laminar
material must be intercalated first to obtain a stable
intercalation compound prior to exfoliation. They have further
believed that the exfoliation of graphite intercalation compounds
necessarily involve high temperatures. It is extremely surprising
for us to observe that prior intercalation is not required of
graphite for exfoliation and that exfoliation can be achieved by
using ultrasonic waves at relatively low temperatures (e.g., room
temperature), with or without prior intercalation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] FIG. 1 A transmission electron micrograph of graphite flakes
obtained by ultrasonic wave-effected exfoliation and
separation.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0035] Carbon materials can assume an essentially amorphous
structure (glassy carbon), a highly organized crystal (graphite),
or a whole range of intermediate structures that are characterized
in that various proportions and sizes of graphite crystallites and
defects are dispersed in an amorphous matrix. Typically, a graphite
crystallite is composed of a number of graphene sheets or basal
planes that are bonded together through van der Waals forces in the
c-axis direction, the direction perpendicular to the basal plane.
These graphite crystallites are typically micron- or
nanometer-sized. The graphite crystallites are dispersed in or
connected by crystal defects or an amorphous phase in a graphite
particle, which can be a graphite flake, carbon/graphite fiber
segment, carbon/graphite whisker, or carbon/graphite nano-fiber. In
the case of a carbon or graphite fiber segment, the graphene plates
may be a part of a characteristic "turbostratic structure."
[0036] One preferred specific embodiment of the present invention
is a method of producing a nano-scaled graphene plate (NGP)
material that is essentially composed of a sheet of graphene plane
or multiple sheets of graphene plane stacked and bonded together.
Each graphene plane, also referred to as a graphene sheet or basal
plane, comprises a two-dimensional hexagonal structure of carbon
atoms. Each plate has a length and a width parallel to the graphite
plane and a thickness orthogonal to the graphite plane. The
thickness of an NGP is 100 nanometers (nm) or smaller and more
typically thinner than 10 nm with a single-sheet NGP being as thin
as 0.34 nm. The length and width of a NGP are typically between 1:m
and 20 .PHI.m, but could be longer or shorter. For certain
applications, both length and width are smaller than 1 .mu.m. In
addition to graphite, graphite oxide and graphite fluoride are
another two of the many examples of laminar or layered materials
that can be exfoliated to become nano-scaled platelets.
[0037] Generally speaking, a method has been developed for
concurrently exfoliating and separating a layered or laminar
material to produce nano-scaled platelets having a thickness
smaller than 100 nm and typically smaller than 10 nm. The method
comprises no intercalation step, although the method is applicable
to intercalated graphite or intercalated graphite oxide compounds
as well.
[0038] Using graphite as an example, the first step may involve
preparing a laminar material powder containing fine graphite
particulates (granules) or flakes, short segments of carbon fiber
or graphite fiber, carbon or graphite whiskers, carbon or graphitic
nano-fibers, or their mixtures. The length and/or diameter of these
graphite particles are preferably less than 0.2 mm (200:m), further
preferably less than 0.01 mm (10:m). They can be smaller than 1:m.
The graphite particles are known to typically contain micron-
and/or nanometer-scaled graphite crystallites with each crystallite
being composed of multiple sheets of graphite plane.
[0039] Although intercalation is not a requirement, we have also
chosen to investigate the exfoliation of intercalated compounds at
low temperatures (e.g., room temperature). Intercalation of
graphite is well-known in the art. A wide range of intercalants
have been used; e.g., (a) a solution of sulfuric acid or
sulfuric-phosphoric acid mixture, and an oxidizing agent such as
hydrogen peroxide and nitric acid and (b) mixtures of sulfuric
acid, nitric acid, and manganese permanganate at various
proportions. Typical intercalation times are between 2 hours and
two days. The resulting acid-intercalated graphite may be subjected
to repeated washing and neutralizing steps to produce a laminar
compound that is essentially graphite oxide. In other words,
graphite oxide can be readily produced from acid intercalation of
graphite flakes. It is important to emphasize that the presently
invented method is applicable to both graphite and graphite oxide
that are either un-intercalated or intercalated.
[0040] The second step of the presently invented method comprises
dispersing laminar materials (e.g., graphite or graphite oxide
particles) in a liquid medium (e.g., water, alcohol, or acetone) to
obtain a suspension or slurry with the particles being suspended in
the liquid medium. Preferably, a dispersing agent or surfactant is
used to help uniformly disperse particles in the liquid medium.
Most importantly, we have surprisingly found that the dispersing
agent or surfactant facilitates the exfoliation and separation of
the laminar material. Under comparable processing conditions, a
graphite sample containing a surfactant usually results in much
thinner platelets compared to a sample containing no surfactant. It
also takes a shorter length of time for a surfactant-containing
suspension to achieve a desired platelet dimension.
[0041] Surfactants or dispersing agents that can be used include
anionic surfactants, non-ionic surfactants, cationic surfactants,
amphoteric surfactants, silicone surfactants, fluoro-surfactants,
and polymeric surfactants. Particularly useful surfactants for
practicing the present invention include DuPont's Zonyl series that
entails anionic, cationic, non-ionic, and fluoro-based species.
Other useful dispersing agents include sodium hexametaphosphate,
sodium lignosulphonate (e.g., marketed under the trade names
Vanisperse CB and Marasperse CBOS-4 from Borregaard LignoTech),
sodium sulfate, sodium phosphate, and sodium sulfonate.
[0042] Conventional exfoliation processes for producing graphite
worms from a graphite material normally include exposing a graphite
intercalation compound (GIC) to a high temperature environment,
most typically between 850 and 1,050.degree. C. These high
temperatures were utilized with the purpose of maximizing the
expansion of graphite crystallites along the c-axis direction.
Unfortunately, graphite is known to be subject to oxidation at
350.degree. C. or higher, and severe oxidation can occur at a
temperature higher than 650.degree. C. even just for a short
duration of time. Upon oxidation, graphite would suffer from a
dramatic loss in electrical and thermal conductivity.
[0043] In contrast, the presently invented method makes use of an
ultrasonication temperature typically lying between 0.degree. C.
and 100.degree. C. Hence, this method obviates the need or
possibility to expose the layered material to a high-temperature,
oxidizing environment. If so desired, the exfoliated product may be
subjected to a subsequent mechanical shearing treatment, also at a
relatively low temperature (e.g., room temperature), such as ball
milling, air milling, or rotating-blade shearing. With this
treatment, either individual graphene planes (one-layer NGPs) or
stacks of graphene planes bonded together (multi-layer NGPs) are
further reduced in thickness (multi-layer NGPs), width, and length.
In addition to the thickness dimension being nano-scaled, both the
length and width of these NGPs could be reduced to smaller than 100
nm in size if so desired. In the thickness direction (or c-axis
direction normal to the graphene plane), there may be a small
number of graphene planes that are still bonded together through
the van der Waal's forces that commonly hold the basal planes
together in a natural graphite. Typically, there are less than 30
layers (often less than 5 layers) of graphene planes, each with
length and width from smaller than 1 .PHI.m to 100 .PHI.m.
High-energy planetary ball mills and rotating blade shearing
devices (e.g., Cowles) were found to be particularly effective in
producing nano-scaled graphene plates. Since ball milling and
rotating-blade shearing are considered as mass production
processes, the presently invented method is capable of producing
large quantities of NGP materials cost-effectively. This is in
sharp contrast to the production and purification processes of
carbon nano-tubes, which are slow and expensive.
[0044] Ultrasonic or shearing energy also enables the resulting
platelets to be well dispersed in the very liquid medium, producing
a homogeneous suspension. One major advantage of this approach is
that exfoliation, separation, and dispersion are achieved in a
single step. A monomer, oligomer, or polymer may be added to this
suspension to form a suspension that is a precursor to a
nanocomposite structure.
[0045] The process may include a further step of converting the
suspension to a mat or paper (e.g., using any well-known
paper-making process), or converting the nanocomposite precursor
suspension to a nanocomposite solid. If the platelets in a
suspension comprise graphite oxide platelets, the process may
further include a step of partially or totally reducing the
graphite oxide after the formation of the suspension. The steps of
reduction are illustrated in an example given in this
specification.
[0046] Alternatively, the resulting platelets, after drying to
become a solid powder, may be mixed with a monomer to form a
mixture, which can be polymerized to obtain a nanocomposite solid.
The platelets can be mixed with a polymer melt to form a mixture
that is subsequently solidified to become a nanocomposite
solid.
[0047] The following examples serve to provide the best modes of
practice for the present invention and should not be construed as
limiting the scope of the invention:
Example 1
Nano-Scaled Graphene Platelets (NGPs) from Natural Graphite
Flakes
[0048] Five grams of graphite flakes, ground to approximately 20
.mu.m or less in sizes, were dispersed in 1,000 mL of deionized
water (containing 0.1% by weight of a dispersing agent, Zonyl.RTM.
FSO from DuPont) to obtain a suspension. An ultrasonic energy level
of 85 W (Branson S450 Ultrasonicator) was used for exfoliation,
separation, and size reduction for a period of 2 hours.
Example 2
Nano-Scaled Graphene Platelets (NGPs) from Natural Graphite Flakes
(No Dispersing Agent)
[0049] Five grams of graphite flakes, ground to approximately 20
.mu.m or less in sizes, were dispersed in 1,000 mL of deionized
water to obtain a suspension. An ultrasonic energy level of 85 W
(Branson S450 Ultrasonicator) was used for exfoliation, separation,
and size reduction for a period of 2 hours.
Example 3
Further Shear Treatment
[0050] A small portion of the exfoliated graphite particles (from
Example 2) was then ball-milled in a high-energy plenary ball mill
machine for 24 hours to produce nano-scaled particles with reduced
length and width.
Example 4
Exfoliation and Separation of Graphite Oxide
[0051] Graphite oxide was prepared by oxidation of graphite flakes
with sulfuric acid, nitrate, and permanganate according to the
method of Hummers [U.S. Pat. No. 2,798,878, Jul. 9, 1957]. Upon
completion of the reaction, the mixture was poured into deionized
water and filtered. The graphite oxide was repeatedly washed in a
5% solution of HCl to remove most of the sulphate ions. The sample
was then washed repeatedly with deionized water until the pH of the
filtrate was neutral. The slurry was spray-dried and stored in a
vacuum oven at 60.degree. C. for 24 hours. The interlayer spacing
of the resulting laminar graphite oxide was determined by the
Debey-Scherrer X-ray technique to be approximately 0.73 nm (7.3
.ANG.).
[0052] Two grams of graphite flakes, ground to approximately 20
.mu.m or less in sizes, were dispersed in 500 mL of deionized water
(containing 0.1% by weight of a dispersing agent, Triton X-100)) to
obtain a suspension. An ultrasonic energy level of 85 W (Branson
S450 Ultrasonicator) was used for exfoliation, separation, and size
reduction for a period of 1 hour.
[0053] The dimensions and electrical conductivity values of the
fully separated graphite flakes or NGPs of Samples A-D are
summarized in Table 1. The electrical conductivity was measured on
"flexible graphite" samples that were prepared by stacking graphene
platelets and compressing the stacked platelets between two platens
in a hydraulic press.
TABLE-US-00001 TABLE 1 Dimensions and electrical conductivity of
NGPs prepared under different conditions. Average platelet Average
platelet Electrical Sample length (.PHI.m) thickness (nm)
conductivity (S/cm) 1 3.5 4.5 3,500 2 4.6 38.6 2,800 3 3.2 19.4
3,200 4 3.3 9.8 37
[0054] It is of significance to note that the presently invented
approach of utilizing a dispersing agent or surfactant in a liquid
medium enables the ultrasonic waves to produce NGPs that are much
thinner (e.g., 4.5 nm in Example 1) as compared to a suspension
without a dispersion agent (e.g., 38.6 nm in Example 2). Without a
high-temperature exposure (hence, relatively oxidation-free), this
new approach also leads to NGPs with a much higher conductivity;
e.g., 2,800-3,500 S/cm as opposed to approximately 1,500 S/cm or
lower commonly reported for commercially available flexible
graphite sheet.
Example 5
NGP Nanocomposites
[0055] Approximately 2 grams of NGPs prepared by spray-drying a
portion of the sample prepared in Example 1 was added to 100 mL of
water and a 0.2% by weight of a surfactant, sodium dodecylsulfate
(SDS), to form a slurry, which was then subjected to
ultrasonication at approximately 20.degree. C. for five minutes. A
stable dispersion (suspension) of well-dispersed nano-scaled
graphite platelets was obtained. A water-soluble polymer,
polyethylene glycol (1% by weight), was then added to the
suspension. Water was later vaporized, resulting in a nanocomposite
containing NGPs dispersed in a polymer matrix.
Example 6
NGPs from Short Carbon Fiber Segments
[0056] The procedure was similar to that used in Example 1, but the
starting material was graphite fibers chopped into segments with
0.2 mm or smaller in length prior to dispersion in water. The
diameter of carbon fibers was approximately 12 .mu.m. After
ultrasonication for 4 hours at 85 W, the platelets exhibit an
average thickness of 9.8 nm.
Example 7
NGPs from Carbon Nano-Fibers (CNFs)
[0057] A powder sample of graphitic nano-fibers was prepared by
introducing an ethylene gas through a quartz tube pre-set at a
temperature of approximately 800 EC. Also contained in the tube was
a small amount of nano-scaled Cu--Ni powder supported on a crucible
to serve as a catalyst, which promoted the decomposition of the
hydrocarbon gas and growth of CNFs. Approximately 2.5 grams of CNFs
(diameter of 10 to 80 nm) were dispersed in water (as in Sample 1).
The sample was then subjected to ultrasonication at 20 EC for two
hours to effect exfoliation and separation, followed by a
mechanical shearing treatment using a rotating-blade device
(Cowles). Fine NGPs with an average thickness of 4.5 nm were
obtained.
Example 8
Graphite Oxide Nano Platelets, their Nanocomposites, and their
Reduced Versions
[0058] The resulting nano platelets obtained in Example 4 were
well-dispersed in water, forming a stable water dispersion
(suspension). Upon removal of water, the nano platelets settled to
form an ultra-thin nano-carbon film (a mat or paper). A small
amount of water-soluble polymer (e.g., poly vinyl alcohol) was
added to the nano platelet-water suspension with the polymer
dissolved in water. The resulting nano platelet suspension with
polymer-water solution as the dispersing medium was also very
stable. Upon removal of water, polymer was precipitated out to form
a thin coating on nano platelets. The resulting structure is a
graphite oxide reinforced polymer nanocomposite.
[0059] A small amount of the nano platelet-water suspension was
reduced with hydrazine hydrate at 100.degree. C. for 24 hours. As
the reduction process progressed, the brown-colored suspension of
graphite oxides turned black, which appeared to become essentially
graphite nano platelets or NGPs.
[0060] Another attempt was made to carry out the reduction of the
graphite oxide nano platelets prepared via the presently invented
method. In this case, hydrazine hydrate reduction was conducted in
the presence of poly (sodium 4-styrene sulfonate) (PSS with
Mw=70,000 g/mole). A stable dispersion was obtained, which led to
PSS-coated NGPs upon removal of water. This is another way of
producing platelet-based nanocomposites.
Example 9
Production of Molybdenum Diselenide Nano Platelets
[0061] The same sequence of steps can be utilized to form nano
platelets from other layered compounds: dispersion of a layered
compound, ultrasonication, and an optional mechanical shear
treatment. Dichalcogenides, such as MoS.sub.2, have found
applications as electrodes in lithium ion batteries and as
hydro-desulfurization catalysts.
[0062] For instance, MoSe.sub.2 consisting of Se--Mo--Se layers
held together by weak van der Waals forces can be exfoliated via
the presently invented process. Intercalation can be achieved by
dispersing MoSe.sub.2 powder in a silicon oil beaker, with the
resulting suspension subjected to ultrasonication at 120 W for two
hours. The resulting MoSe.sub.2 platelets were found to have a
thickness in the range of approximately 1.4 nm to 13.5 nm with most
of the platelets being mono-layers or double layers.
[0063] Other single-layer platelets of the form MX.sub.2
(transition metal dichalcogenide), including MoS.sub.2, TaS.sub.2,
and WS.sub.2, were similarly exfoliated. Again, most of the
platelets were mono-layers or double layers. This observation
clearly demonstrates the versatility of the presently invented
process in terms of producing relatively uniform-thickness
platelets.
[0064] It is clear that the presently invented method is also
applicable to non-graphite, layered materials. Complete exfoliation
and separation are effected at a low temperature using
ultrasonication, optionally followed by a mechanical shearing
treatment. Hence, another preferred embodiment of the present
invention is a method of exfoliating a layered material to produce
separated nano-scaled platelets having a thickness smaller than 100
nm (mostly smaller than 5 nm). The method comprises: (a) dispersing
particles of a layered material in a liquid medium containing
therein a surfactant or dispersing agent to produce a stable
suspension or slurry; and (b) exposing this suspension or slurry to
ultrasonication at an energy level for a sufficient length of time
to produce the desired separated nano-scaled platelets.
[0065] In the aforementioned Example 1, a desired amount of fully
separated graphene platelets were stacked and re-compressed to
become flexible graphite sheets for the purpose of measuring the
relative electrical conductivity of these platelets. The resulting
flexible graphite sheets, having been exposed to no significant
oxidation, exhibit an electrical conductivity typically higher than
3,200 S/cm. By contrast, commercially available flexible graphite
sheets, normally having experienced a high exfoliation temperature
(though possibly under a protective gas atmosphere), exhibit an
electrical conductivity typically in the vicinity of 1,100
S/cm.
[0066] In conclusion, the presently invented method has many
advantages over prior art methods of exfoliating layered materials
for producing nano-scaled platelets. Summarized below are some of
the more salient features or advantages: [0067] (1) The present
method is versatile and applicable to essentially all layered
materials including, but not limited to, carbon- or graphite-based
layered materials. [0068] (2) The method does not involve a high
exfoliation temperature (e.g., typically below 100.degree. C.) and,
hence, avoids undesirable high-temperature chemical reactions
(e.g., avoids oxidation of graphite). The resulting NGPs exhibit
excellent conductivity. [0069] (3) The prior art step of
intercalation, which typically involves using an undesirable acid
such as sulfuric and nitric acid, can be avoided in the presently
invented method. Hence, this is a much more environmentally benign
process. This method is applicable to a wide range of liquid media
(water, organic solvents, monomers, oligomers, etc.). Exfoliation,
separation, and dispersion are essentially combined into one step.
[0070] (4) This method is amenable to the preparation of various
precursor forms (e.g., suspension, paper, mat, thin film, and
lamina) to nanocomposites.
* * * * *